Ms. Tymoshenko's allies said Tuesday she will not concede until appeals have run their course and recounts have taken place at disputed polling stations.

With virtually all votes counted, official tallies show Mr. Yanukovych with more than a three percent lead over Ms. Tymoshenko, the country's current prime minister.

Reports of the pending legal challenges came hours after the Ukrainskaya Pravda newspaper quoted Ms. Tymoshenko as saying late Monday she would never acknowledge a Yanukovych win.

Unlike past Ukrainian presidential polls, international monitors have praised the vote as being free and fair.

The United States has also endorsed the vote. The U.S. Embassy said Tuesday the election was another step in the consolidation of Ukraine's democracy.

Supporters of Mr. Yanukovych rallied for a second day Tuesday outside the Central Election Commission in Kyiv to back the leader of the Party of Regions.

Yields on Ukrainian bonds fell Tuesday after the news of the Tymoshenko challenges. Analysts say a prolonged post-election battle could worsen the already battered Ukrainian economy.

Ms. Tymoshenko has remained silent after European poll monitors Monday declared the election transparent and fair. The Organization for Security and Cooperation in Europe endorsed the poll as "an impressive display of democratic elections."

Mr. Yanukovych has already claimed victory in what analysts both inside and outside Ukraine describe as an astonishing comeback for a candidate and former prime minister who fell from grace with Ukrainian voters in 2004.

Six years ago, official results showed the pro-Russian Mr. Yanukovych winning the presidency over pro-Western Viktor Yushchenko. But huge protests known as the "Orange Revolution" over reports of massive vote fraud led to a new election won by Mr. Yushchenko.

President Yushchenko took office in January 2005 with Ms. Tymoshenko as his prime minister. Their partnership later disintegrated in disputes over presidential powers, and both ran for president this time around. Mr. Yushchenko finished well behind both Mr. Yanukovych and Ms. Tymoshenko in the first round of voting, and was not eligible for the runoff ballot.